A strategy for approaching the therapy of human tumors with internally administered Auger and alpha-emitting radionuclides is being developed. The radiotoxcities of several radionuclides currently used as radiopharmaceuticals in clinical practice are being investigated in cultured mammalian cell lines. Selenium-75, as the amino acid analogue selenomethione, is being compared with sulfur-35 and tritiated methione. The toxicity of thallium-201 is studied in relation to rubidium-86 and potassium-42/43. Iodine-125 has been incorporated into antipyrine and certain DNA binding agents and its radiotoxicity will be determined shortly both in vitro and in vivo. Finally, the radiotoxicity of Astatine-211 in its free anionic astatide form is being investigated.